1. Field of the Invention
The present invention relates to valves associated with microfluidic assemblies, and more specifically, to valves integrally associated with microfluidic assemblies adapted to transport liquid samples for analytical purposes.
2. Description of the Prior Art
A variety of analytical instruments are used to characterize liquid samples containing an analyte of interest, particularly in the context of assays directed to real-time detection of biomolecular interactions. For example, the study of real-time biomolecular interactions through use of “biosensors” are now of fundamental importance in many fields, including biology, immunology and pharmacology. In this context, many biosensor-based analytical instruments include “microfluidic structures” adapted to transport one or more liquid samples through an interaction or a detection zone. Such microfluidic structures generally include a block unit that has multiple internal channels, inlet and outlet ports, pumps and valves; all of which operate in concert to flow small volumes of liquid sample and various other buffers and reagents through one or more interaction and/or detection zones.
An exemplary microfluidic structure for such liquid handling may be illustrated in the context of biosensors that use surface plasmon resonance (SPR) to monitor the interactions between an analyte and a ligand bound to a solid support. In this regard, a representative class of biosensor instrumentation is sold by Biacore AB (Uppsala, Sweden) under the trade name BIAcore® (hereinafter referred to as “the BIAcore instrument”). The BIAcore instrument includes a light emitting diode, a sensor chip covered with a thin gold film, an integrated microfluidic cartridge and photo detector. Incoming light from the diode is reflected in the gold film and detected by the photo detector. At a certain angle of incidence (“the SPR angle”), a surface plasmon wave is set up in the gold layer, which is detected as an intensity loss or “dip” in the reflected light. The theoretical basis behind the BIAcore instrument has been fully described in the literature (see, e.g., Jönsson, U. et al., Biotechniques 11:620–627, 1991).
More specifically, and as best shown in FIG. 1 (prior art), a representative BIAcore instrument 100 comprises a source of light 102, first lens means 104 for directing a transversely extending convergent beam 106 toward a prism 108 whereby the beam is focused in the bottom surface of the prism 108 to thus form a streak 110 of light. Rays of light reflected from the sensitized surfaces are imaged via an anamorphic lens system 112 on a two-dimensional photodetector device 114. The electronic signals created by the photodetectors are processed in an evaluation device 116 in the form of a computer.
By means of the prism 108 and an opto-interface 118 light from streak 110 is directed to a sensor unit 120 which lies in contact with a number of parallel, upwardly open portions 122A–D of flow channels 124A–D, respectively; only one of which, 124A, is shown. The flow channels form part of a block unit 126 for liquid handling, this block unit is shown with schematically indicated inlet connection tubes 128 and 130 and outlet connection tubes 132 and 134. A more complete description of this representative BIAcore instrument including its microfluidic block unit for flowing solutions therein may be found in U.S. Pat. No. 5,313,264, which is incorporated herein by reference in its entirety.
As more fully described in U.S. Pat. No. 5,313,264, and as also best seen in FIG. 1 (prior art), the upwardly open portions 122A–D of flow channels 124A–D (only flow channel 124A is shown) correspond to a first layer 136 of a sealing elastomer material (e.g., silicone rubber or the like) that has a number of cuts or slits extending therethrough. The first layer 136 has been cast onto a plateau 138 which is integral with a base plate 140. The base plate 140 is preferably a solid member made of, for example, plastic, metal, ceramics, or the like.
As best seen in corresponding FIGS. 1A and 1B, a second layer 142 of an elastomer material (e.g., silicone rubber or the like) has been applied by, for example, casting to the underside of base plate 140. The second layer 142 is provided with a system of flow channels or conduits formed by casting. A third layer 144, preferably of the same material as that of second layer 142, has been cast onto a support plate 146 made of a solid material (preferably made of the same material as that of base plate 140).
In view of the foregoing description, it will be readily understood that when the BIAcore instrument 100 is in an operable configuration such that the sensor unit 120 is pressed against first layer 136 by the opto-interface 118, the upwardly open portions 122A–D in first layer 136 will be sealed in liquid-tight relationship against sensor unit 120 and form four flow cells. For sake of simplicity, these four flow cells are also designated 122A–D.
Moreover, in operation, a liquid sample is made to flow through one or more of the flow cells 122A–D. More specifically, a pump (not shown) pumps the liquid sample to inlet tube 128, through an inlet channel 148, through an open valve 150, and then through a primary channel 152 having a fixed and well-defined volume, until it reaches a closed valve 154. The closed valve 154 directs the liquid sample into a waste channel 156 communicating via outlet connecting tube 134 with a disposal receptacle 158.
Next, a valve (not shown) at the upstream end of waste channel 156 is closed, and at the same time valve 150 is also closed. The liquid sample in the primary volume is now ready to be pumped into the flow cell 122A. This is done with the aid of an eluent solution 160 which is pumped by a pump 162 through inlet tube 130 to an eluent conduit 164 ending in a valve (not shown) which is now opened together with valve 154. Continued pumping of the eluent solution 160 causes the advancing eluent solution to press forward against the primary volume of the liquid sample and force it to advance upwardly through a riser duct 166 in the plateau 138, and then into flow cell 122A, and then down through a second riser duct 168 and out through an exhaust duct 170 and an outlet tube 132. From the outlet tube 132, the sample liquid followed by the eluent solution is directed to a waste disposal receptacle 172. When the sample liquid, which has a predetermined volume and/or flow rate, is flowing along flow cell 122A, the chemical interaction between the sample liquid and the sensing surface of the sensor unit 120 is optically detected and analyzed.
One aspect associated with the above-described microfluidic structure, however, lies with the second elastomeric layer 142 (FIGS. 1A and 1B), which elastomeric layer forms part of the valves. In general, the elastomeric layer has low chemical resistance, and may have high permeability with respect to certain gases and small molecules. Both of these attributes are less than optimal in certain embodiments. Accordingly, there is a need in the art for improved microfluidic structures adapted to transport liquid samples for analytical purposes. The present invention fulfills these needs, and provides for further related advantages.